Abstract
Purpose :
The strength of visual responses in primary visual cortex varies with stimulus luminance and size. However, the relative contribution of ON and OFF cortical pathways to signaling luminance remains poorly understood. Here, we investigated how the responses from ON and OFF cortical neurons change with the spatial structure and brightness of the stimulus.
Methods :
We measured the responses from ON and OFF neurons in cat visual cortex to light-dark pattern stimuli (white noise, gratings) and light or dark homogenous surfaces. Cortical neurons were classified as ON or OFF based on the contrast polarity ratio of their maximum response to light and dark small features, CP = (light – dark) / (light + dark), where CP<0 indicates OFF pathway dominance and CP>0 indicates ON pathway dominance.
Results :
When stimulated with patterns, ON and OFF neurons had similar response strength (42.9 ± 1.1 vs. 41.9 ± 1.0 spk/s, p=0.30, Wilcoxon test). OFF neurons responded to small dark stimuli aligned with their receptive field centers, and were suppressed by small light stimuli, and the opposite was found for ON neurons. As stimulus luminance increased, the responses from ON and OFF neurons became stronger (R2=0.84, p<0.001) and ON neurons increased their ON dominance while OFF neurons increased their OFF dominance, a change that may help to distinguish better small dark stimuli from small light stimuli. Surprisingly, both ON and OFF neurons showed strong spatial summation to large bright surfaces and strong spatial suppression to large dark surfaces (dark vs. light size suppression ratio; OFF: 0.46 ± 0.03 vs. 0.16 ± 0.03; ON: 0.39 ± 0.05 vs. 0.08 ± 0.04; p<0.001, Wilcoxon tests). Additionally, as stimulus luminance increased, cortical responses from both ON and OFF neurons, and the combined ON-OFF response, became stronger.
Conclusions :
Our results reveal a cortical mechanism for stimulus size-luminance interactions that may underlie brightness perception. Brightness of small features is signaled with an increase in ON pathway strength but brightness of large surfaces is associated with a strong ON-OFF combined response. This is explained if both ON and OFF neurons have opposite receptive field centers but the same ON extended surrounds. With this structure, small features drive opposite ON and OFF center responses while large surfaces drive the same extended surround, increasing response to light surfaces and suppressing response to dark surfaces.
This is a 2021 ARVO Annual Meeting abstract.